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Researchers View Planets Orbiting Nearby Stars

By Guy Gugliotta
Washington Post Staff Writer
Wednesday, March 23, 2005; Page A07

Astronomers have caught their first direct glimpse of planets orbiting nearby stars, detecting infrared light from two "hot Jupiters" with the help of NASA's infrared Spitzer Space Telescope.

Two separate research teams used the same method to observe the planets, measuring the light from the star and the planet together, then watching the star "dim" as the planet passed behind it in a celestial event known as a "secondary eclipse."

"We've entered a new regime," said Carnegie Institution astrophysicist Alan Boss, commenting on the finds from NASA headquarters during a telephone news conference. "It really whets our appetite. Hot Jupiters are interesting, but we want to find warm Earths, as well."

But Boss and discovery team leaders Drake Deming, of Goddard Space Flight Center, and David Charbonneau, of the Harvard-Smithsonian Center for Astrophysics, said Spitzer was not powerful enough to detect "rocky" planets as small as Earth. NASA has planned missions that can do this, perhaps in 10 years.

Still, the announcement yesterday marked a watershed in the blossoming field of detecting "extra-solar planets," which began 10 years ago when the first such planet was found. Since then, about 150 have been discovered.

All of them, however, were detected indirectly, either by observing a star's "wobble" as it was tugged by the planet's gravity, or because the star's light dimmed slightly as the orbiting planet passed in front of it.

Deming's team, whose research will be published tomorrow in the scientific journal Nature, analyzed the planet HD 209458b, orbiting a star in the constellation Pegasus, 153 light-years away.

Charbonneau's team studied a planet known as TrES-1 in the constellation Lyra, 500 light-years away. The team's report will appear in the June 20 edition of the Astrophysical Journal.

Both planets are "hot Jupiters," extremely large planets orbiting very close to their parent stars. HD 209458b is 4.7 million miles from its host, with a year that lasts 3.5 Earth days. TrES-1 is 4 million miles from its star. (By comparison, Earth is 93 million miles from the sun.) The new research made it possible for the research teams to finally confirm that the "hot Jupiters" were really hot. The infrared measurements showed that HD 209458b has a temperature of 1,574 degrees Fahrenheit, while TrES-1 is at 1,450 degrees Fahrenheit.

And both planets were already well-known, Charbonneau added in a later telephone interview. In fact, a team led by Charbonneau made HD 209458b the first extra-solar planet ever detected using the transit method.

"We have the wobble of the stars, and we have the transit observations," he said.

Until now it has not been possible to directly observe such planets, because the stars outshine them in visible light by a factor of 10,000, making them impossible to see. In infrared light, however, a star is only 400 times as bright, and astronomers can separate a planet from its host.

Spitzer, launched in 2003, is able to ascertain precise variations in the star's and planet's heat signatures without distortions from Earth's atmosphere.

"In a thermally stable environment, you can measure the total brightness" of the planet and star, when the planet is alongside its host, Deming said.

Then, as the planet passes behind the star in an eclipse, the telescope measures the drop in energy. Subtract the star from the total, and the difference is the amount of light from the planet.

"It's the infrared equivalent of making an image," Deming said.

Both planets are similar in size to Jupiter, but astronomers had noted that HD 209458b was "bloated," with a radius 30 percent to 40 percent larger than Jupiter's.

Researchers suggested the distortion occurred because the planet's orbit is elliptical rather than round, causing the star to exert a tidal force on the planet. Earth's tides are caused by the interaction of the Earth, the moon and the sun.

But HD 209458b's orbit was almost perfectly circular, leaving astronomers to scratch their heads. "Drake dashed the hopes of all those theorists," Charbonneau said.

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